This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The eukaryotic DNA polymerase [unreadable] (Pol [unreadable]) participates in genome replication, homologous recombination, DNA repair and damage tolerance. Human Pol [unreadable] consists of four subunits: p125, p50, p66, and p12. The largest catalytic subunit contains the polymerase and 3'[unreadable]5'exonuclease active sites domains. No catalytic activity is associated with the auxiliary p50, p66, and p12 subunits, and they are thought to play a regulatory role, stimulate the polymerase activity of p125 by mediating additional interactions with PCNA, and stabilize the entire Pol [unreadable] complex. p50 serves as a scaffold for the assembly of Pol [unreadable] by interacting simultaneously with all of the other three subunits. In addition, p50 is also involved in the recruitment of several proteins regulating DNA metabolism, including p21, PDIP1, PDIP38, PDIP46 and WRN. The parts of p50 responsible for interactions with p66, p125 and p12 have not been defined. Using two-hybrid screening, the human p66 has been shown to contain p50- and PCNA-binding domains within the 144 N- and 20 C-terminal amino acids, respectively. Interestingly, many essential functions of Pol [unreadable], including the regulation of replication, TLS and BIR, are mediated by the p66 subunit and thus apparently depend on the interaction between its p50 and p66 subunits. The goal of this subproject is to study the complex between the p50 subunit and the 144 amino acids N-terminal domain of the third p66 subunit (p66N) of human Pol [unreadable].